Method and apparatus for cryogenic tank warm-up

ABSTRACT

A warm-up system for liquefied gas storage containers is operable as a closed cycle and comprises a reservoir containing a liquid heat transfer medium, pumping equipment for transferring the liquid from the reservoir to a vaporiser where the liquid is vaporised at an elevated pressure, and heat exchange elements within the storage containers for indirect heat exchange with the cold of said containers. The heat transfer medium for LNG is preferably a petroleum hydrocarbon such as isopentane and the vapour fed from the vaporisor may be adjusted to balance the heating rates of the containers.

United States Patent 1 [111 3,867,818 Tornay Feb. 25, 1975 METHOD AND APPARATUS FOR 1,908,552 5/1933 Randel 237/9 R CRYOGEN1C TANK WARMJJP 3,392,537 7/1968 Woerner 62/50 3,670,517 6/1972 Nonnecke 62/45 Inventor: Edmund George Tornay, London,

England Conch International Methane Limited, Nassau, Bahamas Filed: Oct. 30, 1972 Appl. No.: 301,847

Assignee:

Foreign Application Priority Data Nov. 17, 1971 Great Britain 53481/71 References Cited UNITED STATES PATENTS 11/1932 Raymond 237/9 R Primary Examiner-Meyer Perlin Assistant Examiner-Ronald C Capossela Attorney, Agent, or Firm-Max L. Libman [57] ABSTRACT A warm-up system for liquefied gas storage containers is operable as a closed cycle and comprises a reservoir containing a liquid heat transfer medium, pumping equipment for transferring the liquid from the reservoir to a vaporiser where the liquid is vaporised at an elevated pressure, and heat exchange elements within the storage containers for indirect heat exchange with the cold of said containers. The heat transfer medium for LNG is preferably a petroleum hydrocarbon such as isopentane and the vapour fed from the vaporisor may be adjusted to balance the heating rates of the containers.

7 Claims, 1 Drawing Figure 1 METHOD AND APPARATUS FOR CRYOGENIC TANK WARM-UP This invention relates to warm-up systems for insulated storage containers for cryogenic liquids after removal of said liquids.

The invention may be used, for example, in marine tankers having insulated cargo tanks adapted for the trans-shipment of cryogenic liquids such as cold liquefied gases, e.g., petroleum gases such as propane or butane, or liquefied natural gas (LNG). With such tankers it is important for the economics of any trans-shipment operation that its lay periods in port be reduced to a minimum and, from time to time, e.g., for maintenance purposes, it is necessary to warm the tanks to ambient temperature. It is therefore desirable to achieve this warm-up in a minimum time.

According to one aspect of the invention a methodv of warming up an insulated storage container for a cryogenic liquid after removal of said liquid comprises a. providing a source of heat transfer medium which is in a liquid state and which retains its liquid state at an appropriate pressure over a temperature range from ambient to substantially the boiling point of the cryogenic liquid;

b. adding latent heat to the heat transfer medium whereby to vaporise said medium at an elevated pressure; and

c. feeding the vaporised medium at its elevated pressure into the storage container for indirect heat exchange with the cold of said container, said heat exchange causing the heat transfer medium to return to its liquid state substantially at said elevated pressure, said elevated pressure being adjusted such that it provides sufficient head to assist in maintaining the flow of said medium through said storage container.

It will be appreciated that the invention includes within its scope a system for carrying out the above method.

According to another aspect of the invention a warmup system for a cryogenic storage container comprises a reservoir for a heat transfer medium, pump means for feeding the stored heat transfer medium via a vaporiser to a heat exchanger element arranged within the storage container, and a return line connecting the heat exchanger element and the reservoir, the arrangement being such that, in operation, the medium in liquid form is fed to the vaporisor, is vaporised therein at an elevated pressure and is then fed to the storage container for indirect heat exchange therewith.

For LNG, the heat transfer medium may conveniently be a petroleum hydrocarbon such as propane, isobutane, isopentane, propylene, l-butene or 1- pentene or a flourinated hydrocarbon such as monoclorodi-fluoromethane or dichlorodifluoromethane, although preferably isopentane would be used.

The method and system described provides an alternative to the present system for warming up an LNG marine tanker. The present system comprises circulating hot methane gas through the cargo tanks for direct heat exchange therewith via suitable compressors and heat exchangers. The disadvantages of this system are 1. Large compressors and large diameter pipes are required to provide sufficient mass flow through the tanks; and

2. For a large tanker, say of the order of 120,000m or above, the practical limit for warming up the cargo tanks could well be reached and this limit would be likely to be of the order of two days or more. The limiting factor in this case is the size of readily available compressors suitable for installation on board such a tanker. In the case of a 120,000m tanker, a flow of around 35,000 cu. feet per minute would be required for a warm-up in two days and the present maximum size of a single compressor is around 20,000 cu. feet. Thus, two compressors would be required, significantly affecting the cost of the complete cargo handling system.

It is estimated that, using the indirect isopentane system described hereinafter, the cargo tanks could be warmed up over an average range of temperature from 240F. to +40F. in approximately one day. For a 120,00Om tanker therefore, the indirect system described above could be expected to achieve warm-up in half the time compared with the present system. Furthermore, it is estimated that with the indirect system of the invention, economies of perhaps 10 percent of the cost of the complete cargo handling system could be achieved, compared with the present system, in view of the fact that very large compressors are not required.

In order that the invention may be readily understood a warm-up method and system in accordance therewith, suitable for use in a tanker containing a number of LNG cargo tanks, will now be described, by way of example, with reference to the accompanying drawing which is a flow diagram of the system.

lsopentane is stored at ambient temperature and atmospheric pressure in a storage tank 1 and is fed as required into a reservoir 3 via a gate valve 2 and nonreturn valve 2a. The reservoir 3 is connected to a heat exchanger 4 via a line 5 which has included therein a gate valve 6 at the outlet from the reservoir, a pair of pumps 7 arranged in parallel, each having an inlet and outlet valve 7a, 7b, respectively, associated therewith whereby one pump can act as a stand-by whilst the other is operating, and a further gate valve 8 at the inlet to the heat exchanger. The line 5 also includes a portion 5a by-passing the pumps 7, said portion including a globe valve 5b. The heat exchanger 4 has a connection from a steam line 9 via a regulating valve 10, and the arrangement of the heat exchanger is such that, steam fed thereto provides a source of indirect heat for the isopentane liquid passing therethrough. Steam condensate is fed from the heat-exchanger via outlet line 9a. The isopentane, which is vaporized in the heat exchanger 4 is fed from the top of the latter via an outlet line 11 through a gate valve 13 into a vapour header 16 from which vapour is fed to each of the LNG cargo tanks 17. The isopentane in the vapour header 16 is maintained at a set pressure and temperature, the temperature control comprising the regulating valve 10 in line 9 linked to a temperature controller 15, which is connected into the line 11.

One of the LNG cargo tanks 17 is shown enlarged in the diagram and it will be seen that, in this embodiment, each tank houses two separate heat exchanger coils l8 and 19 fed through inlet lines 21, 22 respectively; a gate valve 23 is provided in each said inlet line. The coils l8, 19 are preferably in the form of multiple finned aluminum tubes. The outlets from the coils 18, 19 are fed to a common line 24 and hence via a gate valve 25 to a liquid header 26 for return to the reservoir 3 and are connected to the storage tank 1 via a gate valve 28 and sight glass 29; the storage tank is provided with a nitrogen vent 30 and a relief valve 31. A return line 32 is provided between the reservoir 3 and storage tank 1, which line includes a control valve 33 linked to a level control 34 for the reservoir, to allow for any increase in volume of the isopentane when its temperature is raised during the system operation. The level control 34 is also linked to a low level pump shutdown device 34a. The line 32 includes a cooler 35 utilising sea-water as the cooling medium.

A control valve 36 is provided in the line 5 between the pumps 7 and gate valve 8, which is linked to a temperature differential controller 37 connected between headers 16 and 26, and a further control valve 38 is provided in a by-pass line 39 connecting the vapor header to the line 27, said further control valve being linked to a temperature controller 41 connected into the line 27.

In operation of the warm-up system with the cargo tanks 17 cold and containing a small quantity of LNG, the system pressurised with nitrogen and all valves closed, the sequence of operation for warm-up are as follows 1. Open valves 2 and 28 ready to fill reservoir 3 with isopentane at ambient temperature.

2. Open valves 6 and 8 and appropriate valves 70, 7b

of a pump 7 and start the pump, filling heat exchanger 4 and reservoir 3 with isopentane. Shut valve 2.

3. Set temperature control at a relatively low level, e.g., I80F, on controller to regulate the steam flow such that isopentane vapour is produced at a pressure of approximately 50 p.s.i.g. Open valve 13 and all vapour and liquid valves 23, 25 to and from tank coils l8, l9. Nitrogen is thus displaced by the isopentane to the top of the storage tank 1 and thence to atmosphere via the vent 30.

4. Shut valve 28 when isopentane liquid arrives at sight glass 29.

5. When the temperature in the reservoir 3 steadies, the system is operating normally. The temperature of the system can then be adjusted gradually to a maximum of 320F, the corresponding pressure being approximately 290 p.s.i.g. by appropriate adjustment to the temperature controller 15 which controls regulating valve 10. The vapour valves 23 may then be adjusted to balance heating rates in the tanks 17.

6. When the warm-up is finished, stop pump 7 and shut off the steam in line 9. Also shut the liquid valve 25 and vapour valves 23 to each tank 17, and valve 13, and allow system to cool down.

7. Purge the system with nitrogen. This is achieved by feeding nitrogen under pressure into and through the system at valved tapping points, for example, at

positions indicated by arrows marked N 8. inactivate the temperature controller 15 and drain all steam condensate through line 90.

The valve 36 under control of temperature differential controller 37 is arranged to operate to maintain the minimum sub-cooling of the isopentane passing through cargo tanks 17 required to suppress flashing" in the isopentane liquid lines, whilst the valve 38 in bypass line 39 operates to prevent the temperature falling below the minimum design temperature for the reservoir 3 and pumps 7.

1 claim:

1. A method of quickly warming up an insulated liquid cryogenic storage container after removal of the liquid and while the container is still cold, comprising a. providing a source of heat transfer medium which is in a liquid state at ambient temperature and which retains its liquid state at an appropriate working pressure over a temperature range from ambient to substantially the boiling point of the cryogenic liquid;

b. adding latent heat to the heat transfer medium whereby to vaporise said medium at an elevated pressure; and

. feeding the vaporised medium at its elevated pressure into the storage container for indirect heat exchange with the cold interior of said container, said heat exchange causing the heat transfer medium to return to its liquid state substantially at said elevated pressure, said elevated pressure being adjusted such that it provides sufficient heat to assist in maintaining the flow of said medium through said storage container.

2. A rapid warm-up system for a recently-emptied cryogenic storage container comprising a. a reservoir for a heat transfer medium in liquid form at ambient temperature and pressure,

b. a vaporizer,

c. a cryogenic storage container, and indirect heat exchange means within said storage container,

d. pump means for feeding the stored heat transfer medium in liquid form from the storage container to said vaporizer and thence to said heat exchange means,

e. means for vaporizing the transfer medium in said vaporizer at an elevated pressure and means for feeding said vaporized transfer medium at the elevated pressure to the heat exchanger means in the storage container but out of direct contact with the interior of the storage container for indirect heat exchange therein so as to warm up said container, as the transfer medium is liquefied during said heat exchange,

f. and a liquid-return line connecting the heat exchanger means and the reservoir.

3. A system according to claim 2,

g. including a number of storage containers, each storage container being provided with a heat exchanger element connected into the system via valves and common vapour and liquid headers,

h. and means for operating said valves to balance the heating rates in the storage containers.

4. A system according to claim 3,

i. including a make-up storage tank, means for con necting the reservoir to said make-up storage tank for the heat transfer medium, and a bypass line provided between the reservoir and storage tank, and a control valve in said line linked to a level control for the reservoir, said control valve being operable to cater for expansion of the heat transfer medium as a result of temperature increase during system operation.

5. A system according to claim 4,

j. and a cooler provided in said by-pass line.

6. A system according to claim 5,

k. and a control valve provided in the line between the pump means and the vaporiser and linked to a temperature difference controller connected across the feed and return for the heat exchange element, the controller being set such that, in operation, the control valve operates to maintain sub- 6 troller connected into said return, the controller being set such that, in operating, the control valve operates to prevent the temperature falling below the minimum design temperature for the reservoir and pump. 

1. A method of quickly warming up an insulated liquid cryogenic storage container after removal of the liquid and while the container is still cold, comprising a. providing a source of heat transfer medium which is in a liquid state at ambient temperature and which retains its liquid state at an appropriate working pressure over a temperature range from ambient to substantially the boiling point of the cryogenic liquid; b. adding latent heat to the heat transfer medium whereby to vaporise said medium at an elevated pressure; and c. feeding the vaporised medium at its elevated pressure into the storage container for indirect heat exchange with the cold interior of said container, said heat exchange causIng the heat transfer medium to return to its liquid state substantially at said elevated pressure, said elevated pressure being adjusted such that it provides sufficient heat to assist in maintaining the flow of said medium through said storage container.
 2. A rapid warm-up system for a recently-emptied cryogenic storage container comprising a. a reservoir for a heat transfer medium in liquid form at ambient temperature and pressure, b. a vaporizer, c. a cryogenic storage container, and indirect heat exchange means within said storage container, d. pump means for feeding the stored heat transfer medium in liquid form from the storage container to said vaporizer and thence to said heat exchange means, e. means for vaporizing the transfer medium in said vaporizer at an elevated pressure and means for feeding said vaporized transfer medium at the elevated pressure to the heat exchanger means in the storage container but out of direct contact with the interior of the storage container for indirect heat exchange therein so as to warm up said container, as the transfer medium is liquefied during said heat exchange, f. and a liquid return line connecting the heat exchanger means and the reservoir.
 3. A system according to claim 2, g. including a number of storage containers, each storage container being provided with a heat exchanger element connected into the system via valves and common vapour and liquid headers, h. and means for operating said valves to balance the heating rates in the storage containers.
 4. A system according to claim 3, i. including a make-up storage tank, means for connecting the reservoir to said make-up storage tank for the heat transfer medium, and a by-pass line provided between the reservoir and storage tank, and a control valve in said line linked to a level control for the reservoir, said control valve being operable to cater for expansion of the heat transfer medium as a result of temperature increase during system operation.
 5. A system according to claim 4, j. and a cooler provided in said by-pass line.
 6. A system according to claim 5, k. and a control valve provided in the line between the pump means and the vaporiser and linked to a temperature difference controller connected across the feed and return for the heat exchange element, the controller being set such that, in operation, the control valve operates to maintain sub-cooling of the heat transfer medium at a level required to suppress ''''flashing'''' in the liquid lines of the system.
 7. A system according to claim 6, 